There has been considerable interest in using optical fibres for the measurement of a wide range of physical and environmental parameters, in particular where the inherent properites of optical fibres offer significant advantages. In applications such as structural monitoring, there is a need for distributed sensor systems for the measurement of strain and temperature, particularly at serial locations. Distributed and multiplexed systems are particularly attractive as they offer monitoring of physical parameters along a length of an optical fibre with benefits of high selectivity and small dimensions enabling them to be readily deployed or embodied within the structure.
It is well known to measure or detect the strain in a structure using interferometer techniques to measure the optical path length changes along a length of optical fibre. For example, when a length of optical fibre is subjected to a strain its length increases and thus the optical path for light passing down the fibre is likewise increased.
However, the temperature variation along a length of sensing fibre can also result in changes of the optical path length of the sensing fibre and make it difficult to distinguish the temperature effects from the strain effects. To try to compensate for this temperature effect the temperature can be measured using, for example, a separate fibre or segment of fibre not subjected to the strain field. However this requires the use of extra fibres and, due to the necessary displacement of this fibre or segment of fibre from the fibre used to measure the strain, accuracy cannot be assured.